Conventionally, a centrifugal air blower used, for example, for a vehicle air conditioner has been so constructed that a fan provided with multiple blades (vanes) between a bottom plate fixed to a rotating shaft and an annular rim is housed in a scroll casing to form a spiral flow passage around the fan in this scroll casing. Then, when the fan is rotated by an electric motor, since inside air in a radial direction of the blades is discharged toward the outside in the radial direction, air is sucked in from a suction port formed on one end side in the axial direction of a rotating shaft, and blown out from a blowing outlet formed on a downstream side toward the outside of the scroll casing via a spiral flow passage.
In this case, if a large volume of air flows between the beginning of winding and the end of winding of the spiral flow passage, since the air supply volume will be decreased to cause an increase in specific sound level as well, a tongue part is formed in the scroll casing to suppress the inflow of air from the end of winding to the beginning of winding of the spiral flow passage. Further, a bell mouth curved to introduce air into the fan (impeller) is formed around an inlet (for example, see Patent Document 1).